The first thing we need is a repeater on the third delay-level (two-right-clicks). Install a redstone-lane around the repeater.
Put a block with a redstone-torch on its side in front of the repeater.
Put a button on it‘s front.
Add another repeater.
Add another repeater, which should enter the „locked mode“ automatically.
Build a block behind this repeater with a redstone-torch on it‘s side and connect the signal of this torch with the repeater. This is also the output-lane. Use the button as an input.